The invention relates to powder coating equipment comprising a line for conveyance air and at least one line of supplemental air, both being connected to an injector to pneumatically move coating powder using the conveyance air of the conveyance line and the supplemental air of the at least one supplemental-air line.
As shown by the versatile state of the art, diverse research and development has followed many approaches to attain optimal operational setpoints using simple steps. However success so far has been elusive. The German patent 1 266 685 discloses the basic principle of an injector pneumatically moving coating powder. The European patent document 0 412 289 B discloses an electrostatic powder coating system containing a display for the total volumetric flow in a feed line of compressed air to the conveyance-air line and to the supplemental air line and a pressure regulator in each of a conveyance-air line and a supplemental-air line. When changing the total volumetric flow by adjusting its associated pressure regulator for the purpose of commensurately changing the flow of powder, the entailed change in the volumetric flow may be eliminated again by correspondingly adjusting the pressure regulator of the supplemental air line. The pressure regulators also may be automatically controlled by a microcomputer instead of being controlled manually. A minimum air flow is required in a powder line from the injector to a spray device or to a container in order to avert powder deposits and flow pulsations in the powder line. The powder flow in the powder line should be as constant as possible, i.e., it should not unduly fluctuate. On that account more supplemental air must be added if the rate of conveyance air must be reduced to such an extent when a small powder flow is desired thatxe2x80x94absent this supplemental airxe2x80x94the total air no longer would suffice. Beyond a partial-vacuum zone of the injector wherein the conveyance air aspirates powder, the supplemental air is introduced into the flow of powder/conveyance-air. In a design variation, or additionally, supplemental air also may be introduced into the partial-vacuum zone in order to vary the partial-vacuum generated by the conveyance air. As a result, when reducing the volumetric flow of conveyance air, the volumetric flow of supplemental air will be increased, and vice-versa. A similar system having a pressure regulator in each line of conveyance air and in each line of supplemental air is known from FIG. 4 of U.S. Pat. No. 3,625,404. Furthermore air-dividing valves are known from said US patent and from the German patent document 44 09 493 A which are fitted with a throttling valve in the conveyance-air line and a throttling valve in the supplemental-air line, said valves being mechanically interlinked and adjusted manually or using motors, whereby, as one throttle is being opened wider, the other shall be closed further. FIG. 3 of said US patent shows an air-divider valve of which the valve chamber and the valve seat can be manually set relative to each other to attain a setpoint for one air flow being larger or smaller than the other. However such mechanically interlinked throttling valves incur the drawback that the setpoint of the differential of the volumetric flows of conveyance air and supplemental air shall be applicable only for very specific kinds of powder and only for a very specific configuration of the powder-coating system, so that, when changing to different kinds of powders, or when changing system components affecting flow conditions, and when changing the flow or timing at which the objects to be coated are being moved past a spray device, said setpoint will not be automatically variable, and new settings shall require interrupting the automated coating procedure. Another drawback of this state of the art is that such manually implemented setpoints require considerable operator experience to secure identical setpoints for recurring identical coating processes. The known mechanical air dividers also can be manufactured only with great difficulty to offer the required high accuracy. On the other hand, using pressure regulators instead of such air dividing valves also incurs the drawback that the volumetric air flow and the powder rate are non-linearly related to the pressures of the conveyance air and of the supplemental air.
This problem is solved by inventive coating equipment comprising a container for containing a coating material, an injector communicated with the container, and a conveyance-air line and at least one supplemental air line connected to the injector. The lines supply conveyance air and supplemental air to the injector for pneumatically sucking the coating material from the container into the injector and moving the sucked coating material toward an object to be coated. A plurality of throttles each mounted in one of the conveyance-air line and the at least one supplemental-air line are equipped with adjustment motors for adjusting a cross section of the throttle, and hence, an air flow of the line. The equipment further comprises an electronic control unit that electrically and mutually adjusts the adjustment motors of all the throttles as a function of a setpoint of a volumetric total air flow of the conveyance air and the supplemental air, and as a function of a setpoint of a material rate at which the coating material is to be applied to the object.
This problem is solved by the invention by means of the features of claim 1.
In accordance with the present invention, when an electric stepping motor is used to adjust the throttles, feedback of actual values to the control unit no longer is required for purposes of control or regulation because the control unit intrinsically always knows how many steps the stepping motor has carried out in either direction of rotation. The control unit is wholly electronic and preferably comprises one or several microprocessors.
The control unit is wholly electronic and preferably comprises one or several microprocessors.
The setpoints for the total volumetric flow, for the required rate of powder and other values may be fed to the control unit. Moreover the control unit can be controlled or regulated automatically by an overriding control center or main computer as a function of objects to be coated, for instance when there is a change in the kind of powder, in the desired coating thickness and/or timing or the conveyance rate at which the objects being coated move past a spray device.